Apparatus for controlling coating thickness

ABSTRACT

A blowing nozzle for controlling coating thickness in continuous hot dip metal coating has an elongated orifice arranged in a plane perpendicular to the plane of the vertically moving strip. A deflector is mounted on the upper side of the nozzle for diverting the effluent gas stream downwardly at an angle against the strip. A guard is mounted on the lower side of the nozzle to prevent deposition of coating metal on the nozzle in the vicinity of the orifice and on the deflector plate.

O United States Patent 1151 3,667,425 Bozeman et a1. 1 51 June 6, 1972 541 APPARATUS FOR CONTROLLING 2,733,653 2/1956 Mullen ..1 18/63 x COATING TI'HCKNESS 3,141,194 7/1964 Jester ..118/63 X 3,314,163 4/1967 Kohler.... .....1l8/63 X [72] Inventors: Wrlght D. Bozeman, Calumet, 111.; Clifford 3 499 41 3/1970 Mayhew 1 1 3 X Blackwell, Crown PO11", 3,611,987 10 1971 Kohler et al. ..1 [8/63 [73] Assignee: Inland Steel Company, Chicago, 111. 0

Primary Examiner-Moms Kaplan [22] Flledi Mar- 1, 1 71 Attorney-Hibben, Noyes & Bicknell [2]] Appl. No.: 119,479 ABSTRACT [52] U 8 Cl 118/63 239/510 A blowing nozzle for controlling coating thickness in continu- [51] In. .0 1365c "/06 ous hot p metal coating has an elongated orifice arranged in [58] Fieid 7 I10 a plane perpendicular to the plane of the vertically moving l II 16418 strip. A deflector is mounted on the upper side of the nozzle for diverting the effluent gas stream downwardly at an angle 56] References Cited against the strip. A guard is mounted on the lower side of the nozzle to prevent deposition of coating metal on the nozzle in UNITED STATES PATENTS the vicinity of the orifice and on the deflector plate.

2,679,231 5/1954 Pomper et al. ..1 18/63 X 8 Claims, 5 Drawing figures F- I'- 1 10 g Z3 5 L 1 1 r I i 1 1 g i l a i J/ APPARATUS FOR CONTROLLING COATING THICKNESS This invention relates to improvements in continuous hot dip metal coating of a metallic substrate. More particularly, the invention relates to improvements in the control of coating thickness in continuous hot dip coating of steel strip with a molten coating metal such as zinc, aluminum, lead, tin, or alloys thereof. v

In continuous hot dip galvanizing and aluminum coating of steel strip it has-been customary for many years to pass the steel strip from a preliminary heating and treating step downwardly into a bath of molten coating 'metal and thence upwardly between a pair of exit rolls at the surface of the bath for controlling the thickness of the coating on the strip. Because of certain well recognized disadvantages of the use of coating rolls, an improved technique has been introduced in recent years which consists in passing the strip upwardly from they bath between a pair of blowing nozzles (sometimes referred to as air knives) which direct high velocity sheets or streams of gas at a regulated pressure and temperature against the opposite surfaces of the upwardly moving strip. The gas blowing technique is described, for example, in such prior art patents as U.S. Pat. Nos. 3,406,656, 3,459,587, and 3,449,418 and British Pat. No. 1,071,572.

Although it has been suggested in the prior art that perpendicular impingement of the gas streams against the strip is the preferred mode of operation, it has been recognized that angular or inclined impingement of the gasstreams against the strip may also be used. Accordingly, the gas blowing nozzles disclosed in the prior art are frequently provided with a complex adjustable mounting or rig which permits adjustment of the angle of blowing relative to the vertical strip as well as adjustment of the distances of 'the nozzles from the opposite sides of the strip.

A primary object of the present invention is to provide a novel and improved nozzle structure for achieving angular impingement of the gas stream against the strip.

A further object of the invention is to provide a novel and improved nozzle structure which can be used either for perpendicular impingement or angular impingement without the necessity for complicating rigging and adjustment means.

Another object of the invention is to provide a simplified nozzle structure for angular impingement of the gas stream against the strip which also includes protective means for 45 preventing accumulation of molten metal on the nozzle in the vicinity of the gas outlet.

Other objects and advantages of the invention will be apparentfrom the subsequent detailed description in conjunction with the accompanying drawings, wherein:

FIG. 1 is a generally schematic view of a hot dip coating apparatus utilizing 'a preferred embodiment of the nozzle structure of the present i vention for controlling the coating thickness;

FIG. 2 is an enlarged end elevational view of one of the nozzle structures shown in FIG. 1;

FIG. 3 is a fragmentary perspective view of a deflector plate portion of the nozzle structure shown in FIGS. 1 and 2;

FIG. 4 is an enlarged transverse sectional view of a modified form of the deflector plate; and

FIG. 5 is a schematic view illustrating certain functional characteristics of the nozzle structure.

Although perpendicular impingement of the coating control gas streams against the strip may be useful or desirable in some cases, we have found that from a practical operating viewpoint there are serious disadvantages to this practice. For

example, on any given galvanizing line the operators may be required from time to time to coat coils of steel strip having different widths, and it is not always possible to select the order in which the coils of varying width will be run through the line. Theblowing nozzles extend across the width of the strip and are necessarily of sufficient length to extend beyond,

the edges of the widest strip which the line is capable of handling. Thus,if the nozzles are 72 inches long and a relativenozzles arranged in opposed alignment for impinging the gas streams perpendicularly against the opposite sides of the strip, it will be recognized that molten metal can be blown off one side of the strip at its opposite longitudinal edges and 5 deposited in or adjacent to the portions of the orifice in the nozzles directed perpendicularly against the strip arises from the fact that, as a result of strip mill rolling procedures, one edge of the strip may be slightly longer than the other edge. When the strip is threaded through the line and placed in tension, the relatively short edge will be taut and, move in a straight path but the relatively longer edge may move in a somewhat irregular path which is commonly referred to as a wavy edge. The straight edge of the moving strip is maintained at a substantially uniform distance from the nozzle outlets, but the distance between the nozzle outlets and the wavy edge of the strip will fluctuate with the result that there is a non-uniform blowing effect at this edge and consequent nonuniform control over the coating thickness.

Both of the aforementioned operating problems are alleviated or eliminated if the gasstreams are directed at a generally downward angle from the noules to the strip. Other advantages are also inherent in angular blowing under certain circumstances. I i

In accordance with the present invention, we are able to achieve the benefits of angular blowing without the necessity of providing complexadjustment mechanisms for rotating or angularly shifting the gas nozzles relative to the plane of the strip. Referring to FIGS. 1-3, a hot dip coating line of the Sendzimir type is shown wherein a continuous steel strip 10 is passed downwardly through a protective conduit or snout ll 40 extending from a preliminary furnace (not shown) into a pot 12 containing a bath 13 of molten coating metal such as zinc, aluminum, or alloys thereof. The strip 10 passes beneath a conventional sinker roll 14 and extends vertically upwardly from the surface 16 of the bath. The strip with molten coating metal adhering to both surfaces passes between a pair of opposed gas noules 17 for controlling the thickness of the coating. Thereafter the strip is cooled and recoiled by conventional means (not shown).

Each nozzle 17 comprises a pair of elongated upper and wer body members or noules halves l8 and I9, respectively, which have recessed interiors and are secured together by screws or the like (not shown) to define a hollow chamber 21. .Although not illustrated in the drawing, the chamber 21 is preferably provided with internal baffle means, e.g., as described in US. Pat. No. 3,360,202. The upper nozzle half 18 has a plurality of gas inlet connections, such as the conduit 22, for supplying a suitable blowing gas to the nozzle. The blowing gas may comprise air, steam, inert gas or mixtures thereof, but an inert gas, such as combustion gas, is preferably used and may be mixed with a predetermined amount of steam. In FIG. 1, a gas source or generator is indicated schematically at 23 for supplying the blowing gas at a controlled temperature and pressure to the conduits 22. Each nozzle 17 5 is long enough to extend across the width of the strip 10 and project somewhat beyond the longitudinal edges of the strip. The nozzle halves 18 and 19 have oppositely tapered or inclined'outer surfaces 24 and 26, respectively, which converge to define a narrow nose portion facing the strip 10. A

gasket or shim 27 is interposed between the nozzle halves 18 and 19 and extends around the mating surfaces of the nozzle halves except at the narrow nose portion. Thus, the nozzle halves are spaced apart slightly at the nose portion to define an elongated restricted gas outlet or orifice 28 which directs a ly narrow strip (e.g., 30 inches) is being coated with the gas high velocity stream or sheet of gas toward the strip 10.

As shown in FIG. 1, the nozzles 17 are mounted in opposed relation at opposite sides of the strip 10 with the outlets or orifices 28 disposed in a common plane substantially perpendicular to the plane of the upwardly moving strip 10. Although omitted from the drawing, it will be understood that the nozzles 17 have the usual means for adjusting the distance between each nozzle and the strip. For directing the effluent gas streams angularly against the strip 10, each upper nozzle half 18 has an elongated blade or deflector plate 31 (FIG. 3) detachably secured to its inclined surface 24 by means of a plurality of screws 32 extending through slots 33 in the plate 31 and into the nozzle body. The deflector plate 31 extends the entire length of the nozzle and has a flat side 34 with a tapered or beveled edge portion 36 which projects forwardly of the orifice 28 into the path of the efiluent gas stream from the orifice 28 for deflecting the stream downwardly at an angle toward the strip 10 instead of horizontally or perpendicularly against the strip. The slots 33 permits adjustment of the deflector plate 31 to obtain the desired effect, but in general it is preferred that the plate '31 be positioned so that the corner or juncture between the inner flat side 34 and the tapered edge 36 is aligned substantially at the center line of the orifice 28, as best seen in FIG. 2. The angle of the tapered edge 36 is selected to provide the desired angle of impingement of the gas stream against the strip 10, e.g., about 15 below the horizontal.

In FIG. 4 a modified form of deflector plate 41 is shown which has slots 42 (similar to the slots 33 of plate 31) and has tapered or inclined deflecting surfaces 43 along both edges of the plate instead of only one edge. Thus, the plate can be reversed should one edge become impaired or damaged. Of course, more than two edges could be tapered, and different taper angles can be provided at the different edges.

For uniform coating thickness it is essential to avoid any obstructions in the path of gas flow from the orifice 28 which would cause variations in the gas velocity or flow pattern across the width of the strip. As described more fully in connection with FIG. 5, there is a'tendency for molten metal to be splashed against the nozzles 17 or otherwise carried by turbulent gas currents into the nozzle area and deposited at the orifice 28 or adjacent thereto. In particular, molten metal can deposit and accumulate on the forwardly projecting inclined surface 36 of the deflector plate 31. As a result, the normal smooth profiles of the orifices 28 and the inclined deflecting surfaces 36 may be disturbed to the extent of interfering with the desired uniform blowing effect across the width of the strip. To avoid this difficulty, a splash guard 46 is provided on each nozzle. In the illustrated embodiment of the invention the guard is in the form of an elongated angle member having a leg 47 with a plurality of slots 48 and a leg 49 projecting perpendicularly from the leg 47 in spaced relation from the deflector plate 31. The angle member extends along the length of the nozzle and is detachably secured to the inclined surface 26 of the lower noule half by means of a plurality of screws 51 extending through the slots 48. The position of the guard 46 is adjustable by reason of the slots 48, but it will be understood that the leg 49 should not project forwardly enough to interfere with the flow path of the effluent gas stream from the orifice 28. Preferably, the guard is adjusted so that the outer edge of the leg 49 is substantially in vertical alignment with the outermost edge of the deflector plate 3 1. In other words, the guard 46 and the deflector 31 extend forwardly of the nozzle orifice 28 to substantially the same extent.

FIG. is a schematic illustration of the manner in which the deflector plate 31 and the guard 46 function. As shown, the effluent gas stream spreads as it leaves the restricted orifice 28 but is deflected generally downwardly at an angle from the horizontal by the surface 36 and impinges against the strip over an area designated at 56. The impinging gas stream splits, one part flowing upwardly at a relatively high velocity as indicated by the arrows in the region 57 between the strip 10 and the boundry line 58, and the other part being diverted downwardly at a relatively high velocity as indicated by the arrows in the region 59 between the strip 10 and the boundry line 61. The nozzle 17 is necessarily located a relatively short distance, e.g., 8 to 10 inches, above the surface 16 of the molten metal bath since the gas from the nozzles 17 must be impinged against the strip 10 while the coating metal is still in molten condition so that it can be displaced by the force of the high velocity gas streams. Consequently, the downwardly moving gas in the region 59 is directed against the bath surface 16 at high velocity and is thence deflected laterally (to the left in FIG. 5) thereby creating a region 62 of relatively high turbulence. The gas thus tends to be recirculated upwardly toward the nozzle 17 and entrains a certain amount of low velocity air below and above the nozzle 17 as indicated by the arrows. The turbulent condition existing above the bath surface 16 results in splashing and/or entrainment of molten metal in the gas and air currents, and small amounts of molten metal are thus transported by the gas and air currents toward the forward or nose portion of the nozzle. However, the projecting leg 49 of the guard 46 is disposed in the path of the recirculating gas flow and prevents the coating metal from being deposited on and accumulating on either the face of the nozzle at the orifice 28 or the inclined deflecting surface 36 of the deflector plate 31.

The invention thus permits-a conventional nozzle which is arranged for perpendicular gas impingement to be converted to an angular blowing nozzle by the simple and inexpensive addition of the deflectorplate 31. If for any reason it is desired to use the nozzle for blowing perpendicularly against the strip, the deflector plate 31 is easily removed. Even when the nozzle is used for blowing gas perpendicularly against the strip, the guard 46 is a useful addition to protect the orifice 28 and the adjacent surfaces of the nozzle from being obstructed by deposits of coating metal. However, when the deflector plate 31 is employed, there is an increased tendency for coating metal deposition and accumulation at the nozzle face, and the forwardly projecting portion 36 of the deflector plate affords an additional surface for metal deposits. Therefore, the guard 46 is particularly necessary and desirable in combination with the deflector plate 31. We have found that the combined use of the deflector plate 31 and the. guard 46 permits effective operation over a wide range of line speeds and for all commonly used strip gages with edge build-up difficulties and with a coating thickness variation across the width of the strip of less than 0.10 oz./sq.ft.

We claim:

1. An apparatus for controlling coating thickness in continuous hot dip metal coating of a metal strip by impinging a stream of gas against the strip as it emerges vertically from the coating bath, said apparatus comprising:

an elongated nozzle structure having an elongated on'fice adapted to face the strip and extend across the width of the strip;

a deflector mounted at one side of said nozzle structure and having an inclined deflecting surface portion disposed forwardly of said orifice for diverting the effluent gas stream at an angle from the plane of said orifice;

a guard mounted at the opposite side of said nozzle structure out of the stream path, and projecting forwardly of said orifice for preventing deposition and accumulation of coating metal on said deflector and on said nozzle structure in the vicinity of said orifice; and

said deflector and said guard being detachably mounted on said nozzle structure.

2. The apparatus of claim 1 further characterized in that said deflector and said guard are adjustably mounted on said nozzle structure for regulating the positions thereof relative to said orifice.

3. The apparatus of claim 1 further characterized in that said deflector comprises an elongated plate number extending the length of said nozzle structure and having a tapered edge portion extending beyond said orifice and comprising said deflecting surface portion.

4. The apparatus of claim 3 further characterized in that said plate member has at least a pair of tapered edge portions thereby permitting reversible mounting of the plate member on said nozzle structure.

5. The apparatus of claim 1 further characterized in that said guard comprises an elongated angle member extending the length of said nozzle structure and having one leg portion attached to said nozzle structure and another leg portion projecting from said nozzle structure.

6. The apparatus of claim 1 further characterized in that said nozzle structure is adapted to be mounted with the plane of said orifice substantially perpendicular to the strip, said nozzle structure having oppositely inclined upper and lower surfaces which converge to define an elongated restricted nose portion containing said orifice, said deflector being mounted on said inclined upper surface, and said guard being 7 adapted to face the strip for impinging'a stream of gas against the strip as it emerges from the coating bath, the improvement which comprises a guard mounted on said nozzle and adapted to be disposed between said orifice and the coating bath, said guard projecting forwardly of said orifice for preventing deposition and accumulation of coating metal on said nozzle structure in the vicinity of said orifice. 

1. An apparatus for controlling coating thickness in continuous hot dip metal coating of a metal strip by impinging a stream of gas against the strip as it emerges vertically from the coating bath, said apparatus comprising: an elongated nozzle structure having an elongated orifice adapted to face the strip and extend across the width of the strip; a deflector mounted at one side of said nozzle structure and having an inclined deflecting surface portion disposed forwardly of said orifice for diverting the effluent gas stream at an angle from the plane of said orifice; a guard mounted at the opposite side of said nozzle structure out of the stream path, and projecting forwardly of said orifice for preventing deposition and accumulation of coating metal on said deflector and on said nozzle structure in the vicinity of said orifice; and said deflector and said guard being detachably mounted on said nozzle structure.
 2. The apparatus of claim 1 further characterized in that said deflector and said guard are adjustably mounted on said nozzle structure for regulating the positions thereof relative to said orifice.
 3. The apparatus of claim 1 further characterized in that said deflector comprises an elongated plate number extending the length of said nozzle structure and having a tapered edge portion extending beyond said orifice and comprising said deflecting surface portion.
 4. The apparatus of claim 3 further characterized in that said plate member has at least a pair of tapered edge portions thereby permitting reversible mounting of the plate member on said nozzle structure.
 5. The apparatus of claim 1 further characterized in that said guard comprises an elongated angle member extending the length of said nozzle structure and having one leg portion attached to said nozzle structure and another leg portion projecting from said nozzle structure.
 6. The apparatus of claim 1 further characterized in that said nozzle structure is adapted to be mounted with the plane of said orifice substantially perpendicular to the strip, said nozzle structure having oppositely inclined upper and lower surfaces which converge to define an elongated restricted nose portion containing said orifice, said deflector being mounted on said inclined upper surface, and said guard being mounted on said inclined lower surface.
 7. The apparatus of claim 1 further characterized in that said deflector and said guard extend forwardly of said orifice to substantially the same extent.
 8. In an apparatus for controlling coating thickness in continuous hot dip metal coating of a metal strip comprising an elongated nozzle structure having an elongated orifice adapted to face the strip for impinging a stream of gas against the strip as it emerges from the coating bath, the improvement which comprises a guard mounted on said nozzle and adapted to be disposed between said orifice and the coating bath, said guard projecting forwardly of said orifice for preventing deposition and accumulation of coating metal on said nozzle structure in the vicinity of said orifice. 